This invention relates to manhole covers of the type widely used in street construction. More specifically, the invention relates to manhole covers which may be vertically adjusted by varying the height to conform the top surface of the cover to the level of a road.
Such manhole covers are normally installed flush with the road surface to provide a smooth transition between the road surface and the manhole cover thereby reducing road vibrations during vehicle travel. When it becomes necessary to repair or resurface the road, the height of the manhole cover may have to be adjusted to correspond to the new height of the paving. Raising of a conventional manhole cover involves extensive break-up of the road covering, removal and re-installation of the basic frame of the manhole cover, and re-paving of the area immediately surrounding the cover. This height readjustment involves a great deal of time, labor, and expense.
In an attempt to solve these problems, vertically adjustable manhole covers have been developed to allow easier and faster height adjustment of the manhole cover frame. One such structure, disclosed in West German Application No. P 2446717.2, utilizes a support member and an adjustable member with complementary steps along their circumferences. By moving the adjustable member to relocate its supporting positions to different steps of the support member, the adjustable member can be raised or lowered. This design requires each step of the support member to carry a corresponding step of the adjustable member resulting in quite heavy components. In addition, a separate pin must be provided to lock the adjustable member against rotation within the support member in the upper step positions. Without this locking pin the vibrations caused by vehicular traffic over the manhole cover could eventually cause the adjustable member to slip off its upper supporting steps to a lower position within the support member.
Accordingly, it is a primary object of this invention to provide a new and improved vertically adjustable manhole cover.
It is a further object of this invention to provide a vertically adjustable manhole cover with an adjustable member which is self-locking in each individual support position to prevent rotation relative to the support member.
Another object of the invention is to provide a vertically adjustable manhole cover which is easily castable and the support member is made lighter in weight.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.